The present invention relates to ultrasound cleaning systems, and more particularly, to systems, generators and methods that clean and/or process by coupling multiple frequency sound waves into a liquid to optimize cleaning performance. This application is related to the following U.S. Patents, which are hereby incorporated by reference: U.S. Pat. No. 5,834,871, Apparatus and Methods for Cleaning and/or Processing Delicate Parts, issued Nov. 10, 1998; U.S. Pat. No. 6,002,195, Apparatus and Methods for Cleaning and/or Processing Delicate Parts, issued Dec. 14, 1999; and, U.S. Pat. No. 6,016,821, Systems and Methods for Ultrasonically Processing Delicate Parts, issued Jan. 25, 2000.
There is a 40 year history of multiple frequency cleaning or processing systems. These systems can be organized into several classes of equipment. The first class of equipment consists of a tank holding liquid with two or more transducers (or two or more transducer arrays) that couple sound energy into the tank and each of these transducers (or arrays) is driven by a different generator. Typically all the generators are operated at the same time or there is an overlap in the operating times of the generators so that two or more frequencies are simultaneously put into the tank for at least part of the cleaning or processing cycle. The chronological history of this first class of equipment starts in 1959 with U.S. Pat. No. 2,891,176 where Branson teaches three transducer arrays driven by three generators, the operation periods of these generators overlap in a way to balance the current in a transformer. In 1974 a tank was designed and built at Branson Cleaning Equipment Company that had an array of 25 kHz transducers on the bottom and a second array of 40 kHz transducers on one side; each of these arrays was simultaneously driven by the appropriate frequency generator. Similar systems were designed and built by others in the 1970's, e.g., Blackstone, but no useful application was found for the technology. In 1981 U.K. Pat. No. 2,097,890A taught three transducer arrays driven by three generators on different phases of a three-phase line. In the mid 1990's Amerimade Technology sold a system consisting of a tank with angled walls and two arrays of transducers on different walls, each array was driven by a different frequency generator, one sweeping around 71.5 kHz and the other sweeping around 104 kHz. At around the same period in time, Zenith sold a two array two generator system operating at 80 kHz and 120 kHz called "crossfire" because the different frequencies intersected at 90 degrees. Unlike the earlier 25 kHz and 40 kHz systems that found no useful application, the personal computer industry now existed and these Amerimade and Zenith systems were sold in large volume to the hard disk drive industry. In U.S. Pat. No. 5,656,095 Honda, et al. teaches high frequency transducers and low frequency transducers on the tank where the high frequency transducers are normally driven and the low frequency transducers are driven for short periods of time to intermittently destroy the high frequency bubbles. In U.S. Pat. No. 5,865,199 Pedziwatr et al. teaches two arrays of transducers interspersed on the tank and driven by two different frequency generators. In U.S. Pat. No. 5,909,741 Ferrell teaches two arrays of transducers on different angled walls of a plastic container and driven by different frequency generators.
A second class of multiple frequency cleaning equipment has one array of multiple frequency transducers that couple sound into the liquid in the tank and this array is driven by a pulse or square wave generator or some other form of shock excitation where the generator output is rich in harmonic frequencies or produces a number of integral harmonic frequencies. Multiple resonances in the multiple frequency transducer array are excited by the appropriate harmonics in the generator's output. Therefore, multiple frequencies are simultaneously coupled into the tank from a single transducer array and a single generator or multiple frequencies are combined in the tank. In U.S. Pat. No. 3,315,102 Quint et al. teaches driving a tank with simultaneous multiple frequencies through shock excitation from a spark gap generator. In U.S. Pat. No. 3,371,233 Cook teaches shock excitation of a non-symmetrical transducer to simultaneously produce many frequencies in a tank. U.K. Pat. No. 1,331,100 teaches a non-symmetrical transducer that can simultaneously vibrate at a number of different frequencies and harmonics of these frequencies. When driven by a generator with a harmonic rich output, this transducer will produce simultaneous multiple frequencies. Other transducers capable of multiple frequencies are taught by Thompson and by Goodson in U.S. Pat. Nos. 4,633,119 and 5,748,566 respectively. In U.S. Pat. No. 5,076,854 Honda, et al. teaches that rapid switching to different frequencies shocks the transducer into producing multi-frequencies in between the drive frequencies. In U.S. Pat. No. 5,462,604 Shibano et al. teaches a way to effectively produce square wave drive characteristics in the liquid by driving the transducer with odd integer multiples of the natural resonant frequency of the transducer. One disadvantage to this class of cleaning equipment is that often the resonant frequencies of the resonantor do not correspond with the harmonic peaks of the driving generator, resulting in less than optimum performance from the cleaning equipment.
A third class of multiple frequency cleaning equipment has multiple transducers (or multiple arrays of transducers), each transducer (or array) having a different frequency and a generator with multiple outputs, each output having a different frequency matched to the transducer (or array) to which it is connected. In U.S. Pat. No. 2,985,003 Gelfand et al. teaches a system where the generator can supply a number of single frequencies to the same number of transducers to set up that number of different standing wave patterns. In U.S. Pat. No. 3,746,897 Karatjas teaches a generator that is capable of supplying an integer number of different single frequencies. One specific single frequency is chosen by a switch for operation. Each specific single frequency drives a different transducer designed to operate at that specific frequency.
One disadvantage common to the above-described configurations is that often the intensity of the ultrasound delivered to the component being cleaned is insufficient for the particular, desired cleaning task.
It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art.